Up to the present, the industrially produced polyester molded articles are widely used in various fibers, film and other molded products. Especially, polyester film prepared from aromatic dicarboxylic acid and glycol has excellent mechanical properties such as thermal resistance, anti-tensile strength, elongation, Young's modulus, recovery of elasticity and impact resistance, as well as dimension stability and electric insulating property, so that it is widely usable in various materials for industry and agriculture such as magnetic recording tape, photograph film, insulating material, film to be deposited. In addition, by virtue of its excellent chemical properties such as chemical resistance, weather resistance and water resistance as well as transparency, fragrance-retaining property and gas-tight property, the amount of use as packing materials for food or other goods is increasing.
Conventionally, the industrial process for preparation of polyester film comprises the steps of melt-extruding polyester chips and cooling the extrudate in a cooling drum to prepare a non-crystalline sheet; and elongating and thermally setting the sheet to obtain double-axis elongated film. According to a conventional industrial process for preparing the polyester chips used in the process for preparing polyester film, a dicarboxylic acid (for example, terephthalic acid) and a glycol (for example, ethylene glycol) are directly subjected to esterification by heating at a reaction temperature between 200-280.degree. C., or a dimethylcarboxylate (for example, di methyl terephthalate) and a glycol (for example, ethylene glycol) are subjected to transesterification at a temperature between 140-240.degree. C. in the presence of a catalyst to obtain bis(.beta.-hydroxyethyl)terephthalate and/or low molecular weight condensate (esterified product) thereof, and the product is then heated at a reaction temperature between 260-300.degree. C. in the presence of a polycondensation catalyst under high vacuum to perform a polycondensation, and the polyester polymer is then extruded into cooling water in the form of spaghetti, and the obtained product is cut in a certain length to give polyester chips.
Conventional polyester film, that is prepared by subjecting a series of film-forming process with polyester polymer obtained from aromatic dicarboxylic acid (or dimethylcarboxylate) and glycol, is industrially valuable due to excellent physical properties mentioned above, however, sometimes the application is rather restricted because it has few reactive groups owing to its molecular structure. In other words, polyester film has inferior adhesiveness, dyeability, hygroscopicity and anti-static property to those of polyamides, polyacetates or celluloses.
For improving such deficiencies of polyester film, in particular improper dyeability, various methods--such as adding a copolymeric component to aromatic polyester to form a copolymeric polyester, or incorporating a compound having good affinity to the dye to lower the crystallinity of the obtained polyester film, or strengthen the bonding between polyester and dye--are suggested. However, none of these conventional method can afford sufficient effect, but at least a part or even all of the excellent mechanical properties, physical properties and/or chemical properties of polyester film as described above is deteriorated by lowering of softening point, color change or heterogeneous mixing of said polyester film.
For this reason, several methods are suggested in order to improve dyeability of polyester without separately adding any copolymeric component, but problems still remains in these methods. First, using a dye having relatively small molecular size results in deficient adhesiveness of dye at the time of washing, while adding a carrier in the aqueous dye bath involves increase of production cost. As the process of pretreating with a semi-solvent forms a pretreated molded article having sheath-core structure so that the outer layer can easily absorbs dye to cause heterogeneous dyeing through the direction of thickness. Besides, a solution dyeing method, in which a dye and polyester are mixed in molten state, is suggested. Though the method gives good results, it is not practically useful in manufacturing various products with small scale.
In order to solve the problems of conventional methods mentioned above, the present inventors have continuously performed intensive studies, and as a result, they invented easily dyeable polyester film from polyester homopolymer without separately incorporating copolymeric component, and process for preparing thereof.